I tried this on thegeeke... it didn't work out too well for me in the end.......
I have two versions of this prank. The first makes the hard drive temporarily unusable, so the computer will power up, the screen will work, but it will give an error message that will make it appear that the computer has stopped working. The second makes it so that the computer will not even power up. This variation works well if the victim knows absolutely nothing about computers, but if they know even a little bit, it will make it easier for them to figure out what happened.
Step 1: Unplug the Computer
Step 2: Open Up the Computer
You will want to use a ground strap to make sure you don't damage anything inside the computer with static electricity (computer components can be damaged with as little as 50 V of static electricity and it takes at least 250 V before we can feel it; if I remember correctly). Slip the strap around your hand so that it is on your wrist touching bare skin, then attach the alligator clip to some part of the metal case for the computer.
Step 3: Disconnect the Power From the Hard Drive (version One of the Prank)
Step 4: Disconnect Power From the Motherboard (version Two of the Prank)
Now you will disconnect the two connections that go from the motherboard to the power supply.
Step 5: Clean Up
Put the cover back on the computer, plug everything back in on the outside, remove all traces of yourself, and stand back and watch the fun! :)
Please note that I am not responsible for anything that might happen because of this prank. Use at your own risk.